Calendrical calculators posses the autistic savant skill of being able to calculate the day for any given date. While the may not be able to do relatively simple arithmetic they have this specific ability which, if achieved mathematically, requires complex operations, as O'Connor (1984) explains:

One method involves starting with the last two digits of the year involved, dividing these by four and adding the integer part to the dividend, ignoring the remainder. A number between 0 and 6, representing the indicated month, is then added and, finally the day of the month is added to the running total.This total is then divided by seven, the result ignored but the reminder noted. This remainder is then used to enter a table of days of the week to obtain the answer.